It is a recent tendency in an automotive vehicle to change a mechanical drive system to an electric system driven by a motor for meeting demands for less space, improved manufacturing, improved controllability and the like. As one exemplary system, a shift range switchover mechanism for an automatic transmission system of a vehicle is driven by a motor. This system includes an encoder provided to output a pulse signal at every predetermined angular rotation in synchronism with the rotation of the motor. When a shift range is switched over, a shift range is switched over to a target range by rotating the motor to a target rotation position (target count value) corresponding to the target shift range based on a count value of the pulse signal of the encoder.
According to such a system, as disclosed in JP-A-2004-308752 for example, a butting control is performed. In the butting control, a motor (actuator) is rotated until a shift range switchover mechanism butts a limit position in a movable range (P-wall position or non-P-wall position) to learn the limit position as a reference position. A rotation amount (rotation angle) of the motor is controlled based on the learned reference position as a reference.
In a system, in which a current flowing in each phase of a motor is duty-controlled based on a duty ratio corresponding to a voltage in a butting control, a current value of each phase of the motor varies with coil resistance changes caused by temperature changes, aging changes or the like, even if the voltage is not changed. As a result, it is likely that a torque of the motor varies and precision of learning will be lowered.
One system, which is under study, includes a current sensor for detecting a sum of currents flowing in plural phases of a motor (for example, a current flowing to a junction point of phase coils) and performs a constant current control to control the sum of currents flowing in the phases of the motor to a constant value based on the output of the current sensor in performing the butting control so that a change in the current value of each phase due to temperature change and aging change is suppressed. According to a system, in which a current supply phase of a motor is switched over by a one-two phase current supply method (one-phase current supply and two-phase current supply are switched over alternately among phases of the motor) while performing the constant current control, the current value of the current supply phase changes at every switchover between the one-phase current supply operation and the two-phase current supply operation. This causes changes in the motor torque and hence lowers the precision in the reference position learning.